The present invention relates to a commutator, a direct-current motor provided with the commutator and a manufacturing method of the commutator.
Conventionally, in order to downsize a direct-current motor, it is desired to downsize a commutator provided in an armature. A commutator described in Japanese Laid-Open Patent Publication No. 2005-137193 has a cylindrical insulative holding portion fixed to a rotary shaft of an armature. A plurality of segments are distributed in a circumferential direction on an outer circumferential surface of the holding portion. A tabular short-circuit unit is arranged on one end surface in the axial direction of the holding portion. The short-circuit unit is connected to the segments.
The short-circuit unit is constituted by two short-circuit groups. Each of the short-circuit groups has the same number of outer short-circuit ends as the segments, the same number of inner short-circuit ends as the outer short-circuit ends, and the same number of coupling portions as the outer short-circuit ends. The outer short-circuit ends are arranged in a circumferential direction. The inner short-circuit ends are arranged in a circumferential direction in an inner side of the outer short-circuit ends. The coupling portion couples the outer short-circuit end to the inner short-circuit end positioned so as to be displaced by a predetermined angle in the circumferential direction from the outer short-circuit end. The inner short-circuit end, the outer short-circuit end and the coupling portion are formed on the same plane. Two short-circuit groups are laminated in such a manner that the coupling portions are directed inversely to each other. The outer short-circuit end is electrically connected to the segment. The outer short-circuit ends of two short-circuit groups are brought into contact with each other in a laminating direction. The inner short-circuit ends of two short-circuit groups are brought into contact with each other in the laminating direction. Each of the coupling portions is not in contact with each other in the laminating direction.
A tabular short-circuit unit is used for the commutator in the publication. Accordingly, for example, in comparison with the case that a conducting wire is used as the short-circuit unit, it is possible to make a structure for short-circuiting the segments to be set to the same electric potential small in the axial direction. As a result, it is possible to intend to downsize the commutator provided with the short-circuit unit in the axial direction.
In the commutator in the publication mentioned above, a plurality of segments are firmly fixed to an outer circumferential surface of the cylindrical holding portion. Accordingly, a feeding brush is brought into slidable contact with the outer circumferential surface of the commutator from a radial direction. In this case, the outer diameter of the commutator tends to become smaller at a degree of a layout space of the feeding brush. In the case that a direct-current motor is downsized in the radial direction, the outer diameter of the commutator becomes further smaller. Generally, the dimension in the radial direction of the short-circuit unit is substantially equal to the outer diameter of the commutator. Accordingly, if the outer diameter of the commutator becomes small, the dimension in the radial direction of the short-circuit unit becomes smaller. Therefore, a cross-sectional area orthogonal to a current passing direction of the coupling portion becomes smaller, an electric resistance of the coupling portion becomes larger, and the short-circuit unit tends to generate heat. Accordingly, it is desired to enlarge the cross-sectional area orthogonal to the current passing direction of the coupling portion. In order to enlarge the cross-sectional area of the coupling portion, there are considered to enlarge the dimension in the axial direction of each of the coupling portions, and to enlarge the dimension in the circumferential direction of each of the coupling portion.
However, in the case of enlarging the dimension in the axial direction of each of the coupling portions, a laminating number of the short-circuit groups is increased. In this case, not only the number of components is increased, but also the dimension in the axial direction of the short-circuit unit is increased. As a result, even in the case of using the tabular short-circuit unit, it is hard to downsize the commutator in the axial direction.
Generally, the dimension in the radial direction of the short-circuit unit is substantially equal to the outer diameter of the commutator. It is necessary to make the adjacent coupling portions in the circumferential direction non-contact. Accordingly, it is hard to enlarge the dimension in the circumferential direction of the coupling portions.
An objective of the present invention is to provide a commutator which can enlarge a cross-sectional area in a current passing direction of a coupling portion while suppressing an enlargement of the dimension in the axial direction. Further, an object of the present invention is to provide a motor provided with the commutator. Further, an object of the present invention is to provide a manufacturing method of the commutator.